3,4,3′,4′-biphenyltetracarboxylic dianhydride (hereinafter, may be abbreviated as s-BPDA) and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (hereinafter, may be abbreviated as a-BPDA) are each known as a monomer source material for producing aromatic polyimide. In particular, a polyimide produced from s-BPDA as a monomer component is superior in properties such as heat resistance, electrical insulation, film strength, film dimensional stability, solvent resistance and the like. Accordingly, because of a high demand for s-BPDA, an efficient process for producing a high purity s-BPDA has been intensively investigated.
On the other hand, even if polymerized with the same aromatic diamine, a-BPDA and s-BPDA produce polyimides having entirely different properties. Hence, as to a-BPDA, a high-purity product containing no s-BPDA is needed.
U.S. Pat. No. 4,958,002 (Patent Reference 1) describes a process for obtaining s-BPDA by dehydrating 3,4,3′,4′-biphenyltetracarboxylic acid (hereinafter, may be abbreviated as s-BPTA) in a nitrogen atmosphere at a temperature up to about 280° C. as final stage. In this process, dehydration progresses while crystalline state is maintained during the process. Since a crystalline-powdery product is thus obtained, a highly-pure product can be efficiently produced.
On the other hand, in terms of production of a-BPDA, the present applicant proposed a process for obtaining highly-pure and powdery a-BPDA by dehydrating 2,3,3′,4′-biphenyltetracarboxylic acid (hereinafter, may be abbreviated as a-BPTA) in inert gas atmosphere at a temperature from 180 to 195° C. in JP-A-2006-328040 (Patent Reference 2). Since, this process aims at obtaining a powdery product, however, the process is essentially for a batch operation and is not so suitable for a continuous operation.
As an example for obtaining a-BPDA by a dehydration reaction in a molten state, Patent Reference 2 describes an example using a hot-air-circulating type heater (see the comparative example 2 in Patent Reference 2). Since a usual hot-air-circulating type heater utilizes a tray, this method is also for a batch operation. Although a continuous hot-air-circulating type heater is also available, it has partially open portions and, therefore it has a defect in that sublimates are prone to form and oxidation-degradation of highly-reactive anhydrides (a-BPDA) are prone to take place.
Furthermore, the evaluation by the present inventors revealed that a product suitable for producing polyimide is difficult to produce by the production of a-BPDA in the case of conducting a dehydration reaction in a molten state. In the case of the production of s-BPDA, there is no defect in the quality of the anhydride obtained even if the dehydration reaction is carried out in a molten state by melting the raw material on a tray under a nitrogen atmosphere. In the case of the production of a-BPDA, however, if the dehydration reaction was carried out in a molten state by melting the raw material on a tray under a nitrogen atmosphere, a-BPDA of high quality is not obtained; that is, high-molecular-weight polyamic acid (i.e., polyimide precursor) was not easily obtained from the a-BPDA obtained.    Patent Reference 1: U.S. Pat. No. 4,958,002    Patent Reference 2: JP-A-2006-328040 (US-2006-0247445 A1)